1. Field of the Invention
This invention relates to a rotor blade, specially for a helicopter rotor, this blade being made of plastics strengthened by high-strength synthetic fibers, and to a process for making this blade in a single moulding operation.
2. Description of the Prior Art
Many different kinds of rotor blade, specially for helicopter rotors, are known which are made of plastics strengthened by high-strength synthetic fibers. In the conventional construction the airfoil part of the blade comprises a spar, which is prepared by the separate moulding of layers of filaments impregnated with thermosetting resin, and a rear member formed by a filler of a lightweight cellular material. Consecutive steps in manufacturing the blade comprise covering the above mentioned members with an envelope or covering made of a fabric of fibres pre-impregnated with a polymerizable synthetic resin, then possibly adding a leading-edge cap or the like made of metal or of a resilient substance, such as polyurethane, and a trailing-edge member, and finally assembling all these members by sticking and hot polymerisation.
French Patent No. 1,255,075 to Societe Grenobloise d'Etudes et d'Applications Hydrauliques (SOGREAH), issued on Jan. 23, 1961, discloses means to increase the rigidity and strength of reinforced plastics blades of this kind and to reduce their weight. This blade mainly comprises a solid central spar as a strengthening framework and one or more shaped members which are stuck to the spar. The spar core is first prepared by polymerizing together cores which are made of an expanded material and which are separated by glass fabric partitions. Rovings of fibres are then disposed lengthwise in two layers one above another and anchored to a connecting flange to form the sill of the spar, whereafter the system comprising the core and the sill, strengthened by a lateral binding with laminated fabrics, is polymerized; finally, the blade covering or envelope is made by sticking transverse pieces of plastics fabric to the spar after it has been provided with appropriate ribs and strengthenings.
U.S. Pat. No. 3,237,697 to Ford and Tarczinski, issued on Mar. 1, 1966, also discloses a plastics blade comprising a core, a spar assembly and a leading-edge assembly, which is prepared beforehand, and then assembled together with the blade covering, consisting of sheets or the like of reinforced plastics. The spar assembly, which is also moulded separately, consists of an inner support which is made of a lightweight cellular substance which is covered by two rows of plastics sheets, with reinforcement by filaments which extend lengthwise in the direction of blade span. The spar assembly is then placed for polymerization in the mould in which the leading-edge assembly and the blade covering were first moulded. In a final phase the system placed in the mould is joined to the blade core of cellular material and to edging members.
U.S. Pat. No. 3,321,091 to Dmitroff and Fox, issued on May 23, 1967, also discloses the manufacturing of a glass fibre reinforced plastics blade. The blade root part consists of a stack of metal plates alternating with glass fibre cloth sheets. The blade transition part comprises of flat core prepared beforehand with a plastics reinforced by a glass fibre roving. The flat core extends with a different shape in the airfoil part of the blade and is disposed along the leading edge and intimately follows the shaped part thereof, this core being the strengthening member of the blade. The rear part of the blade is formed with cavities bounded by glass fibre cloth partitions extending from one surface to the other; these partitions are formed by means of formers during preparation of the blade in metal moulds. When vacuum-injected into the moulds the plastics impregnates the glass fibre cloths.
Most of these known blades are therefore manufactured in a number of steps or phases: any mention of a single moulding operation in the above mentioned patents does actually refer to the final moulding of components previously produced by being moulded separately, sometimes by means of formers. The kind of spar used in these known blades to increase blade strength actually exists only in the small proportion of the span between the connecting member and the airfoil part of the blade, where the cross-section of the spar changes from a D-shape to a C-shape.
It must be also appreciated that the stiffness characteristics of a rotor blade in its plane and in a plane perpendicular thereto are to be so determined that the beat and drag frequencies of the rotating blade differ from the frequency corresponding to the angular velocity of the rotor and from the harmonics of such frequency.
Many experiences with blades of this kind fitted to rotors which may or may not have beat and/or drag articulations have further convinced the Applicants that to reduce vibrations it is very advantageous to suppress any coupling between the beat or drag movements of the blade and blade twisting and therefore to give the blade a very high torsional stiffness.
U.S. Pat. No. 3,782,856 to Salkind and Reinfelder, issued on Jan. 1, 1974, discloses means for increasing the torsional stiffness of a helicopter blade. The blade thus disclosed consists of two strengthening beams each embodied by a central honeycomb core; stuck to the outside surface thereof is a spar in the shape of a flattened U, such spar extending longitudinally and consisting of high-strength fibres, e.g. of graphite or boron, or by composite fibre systems comprising metal fibres. Each beam also has on either side of the central core leading-edge counterweights, a front filler and a rear filler. The two joined-together beams are enveloped or encased in a skin consisting of alternate layers, one-third of which are formed by sheets of glass fibres parallel to the blade longitudinal axis while two-thirds are formed by sheets of graphite fibres disposed at angles of +45.degree. or -45.degree. to the latter axis. The whole forms a "torque-tube" to the rear of which unitary separate pockets stuffed with a lightweight material are stuck. However, the manufacture of this blade comprises the following discrete steps:
1. Preparing the first half of the torque-tube, comprising placing the skin and spar materials in a mould and polymerizing them, then inserting the central core, the counterweights and the fillers and sticking them, then machining the outside surface of this first half of the torque-tube;
2. preparing the second half of the torque-tube by the same sequence of operations, and
3. assembling the two tube halves together after any necessary weight corrections, and finally sticking the pockets to the rear part of the tube-torque.
This blade has the disadvantage of being expensive to manufacture since fibres having a high elasticity modulus such as graphite and/or boron fibres are much dearer than ordinary fibres such as glass fibres--ten times dearer in the case of graphite and forty times dearer in the case of boron. Also, these very high elasticity modulus fibres call for a manufacture which is not only more difficult but also costlier, since fabrics produced with such fibres are difficult to coat and to stick, because they have incompatibility with some resins, and their stiffness makes it difficult to position them.
U.S. Pat. No. 3,950,115 to Euler, issued on Apr. 13, 1976, discloses a high strength rotor blade comprising longitudinal rovings forming a loop at the blade root; the rovings are assembled by a compact filling with a lateral extension of a metal mounting member, comprising a socket which is surrounded by said roving loop and through which a metal pin may be engaged to secure the blade to the rotor hub. In spite of one of said rovings being placed at the leading edge of the blade, the latter comprises no means to increase its torsional stiffness.
French Patent Application No. 76.08109 (2,304,512) to Textron Inc., published on Oct. 15, 1976, discloses a composite rotor blade, comprising a substantially C-shaped two-part leading edge member consisting of plastics strengthened by longitudinal fibres, an inner tubular sleeve consisting of plastics strengthened by fibres arranged in layers crossing each other, said inner sleeve being engaged into the rear open channel of said C-shaped leading edge member, an outer tube consisting of plastics strengthened by fibres arranged in layers, crossing each other, said outer tube surrounding said leading edge member and said inner sleeve, a rear member, and a covering jointing together said outer tube and said rear member. With this blade construction, a "torque-tube" is formed only by the inner sleeve; nevertheless, as this "torque-tube" is hollow, and not substantially thicker than the outer tube, whereas its cross-section has a much less area than the cross section of said outer tube, said "torque-tube" only slightly increases the torsional stiffness of this blade. Moreover, its mechanical strength is reduced and its manufacturing is made uneasy because the C-shaped leading edge member has a rear open channel, into which the inner sleeve is to be engaged, so that said C-shaped leading edge member is necessarily a two-part member.